Conventionally, as image heating devices, for which fixing devices are a typical example, contact-heating devices such as heat roller type devices and belt type devices, generally have been used.
In recent years, due to the demand for shorter warm-up time and reduced energy consumption, electromagnetic induction heating, by which rapid heating and high efficiency heating are likely to be attained, are attracting great attention (see JP 10(1998)-123861 A).
FIG. 23 shows a cross-sectional view of an image heating device utilizing the electromagnetic induction, which is disclosed in JP 10(1998)-123861 A. As shown in FIG. 23, a magnetization coil 114 is provided inside a heat-generating roller 112. By this magnetization coil 114 and a core 117, an alternating magnetic field is generated to induce an eddy current in the heat-generating roller 112, thereby heating the heat-generating roller 112. Then, an unfixed toner image 111 formed on recording paper 110 can be fixed after the recording paper 110 has passed through a nip portion formed between the heat-generating roller 112 and a pressure roller 113. Further, an image heating device with a heat-generating roller that is made thin has been proposed, as disclosed in JP 10(1998)-74007 A. FIG. 24 shows this device.
In FIG. 24, reference numeral 310 denotes a magnetization coil, which generates a high-frequency field when a high-frequency current is applied thereto from an inverter circuit, and reference numeral 311 denotes a metal sleeve, which generates heat through electromagnetic induction and is rotated. An external pressure member 313 rotates in arrow direction “a”. The metal sleeve 311, which is held between the external pressure member 313 and an internal pressure member 312, rotates following the external pressure member 313.
A recording paper 314 carrying an unfixed toner image thereon is fed to the nip portion formed between the external pressure member 313 and the internal pressure member 312 in the arrow direction shown in the drawing. The unfixed toner image on the recording paper 314 is then fixed by the heat from the metal sleeve 311 and the pressure from both the pressure members 312 and 313.
Further, to prevent electromagnetic induction heating from being performed while the metal sleeve 311 is at rest, a heating signal for the inverter circuit is set to be a logical product of an operation signal and a heating signal from a drive motor for rotating the external pressure member 313.
In image heating devices utilizing such electromagnetic induction, a heat-generating member such as a heat-generating roller or the like is directly heated through electromagnetic induction. Such image heating devices thus can attain higher heat-exchanging efficiency as compared with those using a halogen lamp for heating, so that the surface of a fixing roller can be heated up to a fixing temperature rapidly with a smaller power.
However, the image heating device in which a normal metal heat-generating roller is simply heated through electromagnetic induction cannot attain remarkably reduced warm-up time as compared with conventional image heating devices using a halogen lamp for heating.
Further, if a heat generating roller is made thinner to decrease the thermal capacity for shortening warm-up time, it becomes difficult to control the temperature of the roller.
JP 8(1996)-137306 A has proposed an image heating device using a belt with a smaller thermal capacity for shortening warm-up time. In this image heating device, the belt formed of a conductive material is heated through electromagnetic induction and the belt itself thus can be heated rapidly. However, since the thermal capacity of the belt is too small, the heat generated by the belt is removed by a tension roller and an oil roller, which brings about a problem that it is difficult to raise the temperature of the entire system.
For shortening warm-up time, a rotating operation of the heat-generating roller is generally started after the heat-generating roller is heated up to a predetermined temperature. However, since the roller can be heated rapidly according to the electromagnetic induction heating, if the heat-generating roller at rest is heated in the image heating device with a small thermal capacity, an abrupt temperature rise may occur at a portion of the heat-generating roller. This may result in deterioration of the belt, an elastic material provided on the belt, and the like.
Especially in an image heating device performing heating with a heat-generating roller and a heat-resistant belt looped around the roller, the temperature of the heat-generating roller is made too high by the rapid heating, resulting in permanent set of the heat-resistant belt in accordance with the curvature of the roller. It is to be noted here that this problem seldom occurs in the case of a conductive belt and never occurs in an image heating device in which a straight portion of the belt is heated. This problem occurs only in an image heating device in which a heat generating roller is heated and the heat from the roller is conveyed by the belt formed of a resin.
From the viewpoint of saving energy, it is preferable that a heat-generating member in an image heating device is heated only when the device is used. Image heating devices of heat roller type generally include a heat-generating member in a nip portion. However, in image heating devices of the belt type, a heat-generating member is away from a nip portion, resulting in time lag between the temperature change in the heat-generating member and in the nip portion.
In addition, in the image heating devices in which the heat-generating member is away from the nip portion, the heat from the belt, which has been heated by the heat-generating member, is not only consumed for melting toner on recording paper but also for heating a pressure roller and a fixing roller. The pressure roller and the fixing roller are heated by removing the heat from the belt. Accordingly, the amount of the heat removed by these rollers depends on the amount of the belt that has been passed, i.e., the process speed. The heat removed by these rollers is not directly involved in the fixing operation. Therefore, it is necessary to minimize the amount of this wasted heat for performing the fixing operation quickly.
In an image heating device including a magnetization coil and a rotatable conductive heating element, if the device is configured so that the conductive heating element is heated through electromagnetic induction only when the element is rotating, the magnetization coil should magnetize the element after a rotating operation of the element is started. Otherwise, the temperature of the element is only partially made high, resulting in uneven temperature distribution. Although this configuration permits a relatively short warm-up time, it is necessary that the conductive heating element keeps residual heat during a standby period for immediately satisfying the user's demands for printing. However, in the image heating device with this configuration, a rotating operation of the conductive heating element has to be performed for heating the element, which brings about a problem that the element needs to be kept rotating even in the standby period. Besides, since the conductive heating element is heated rapidly, it is difficult to maintain the element at low temperatures.
In the belt-type image heating device, if a temperature sensor is provided on the surface of the belt, the sensor is liable to damage the surface of the belt, thereby reducing the life of the belt. On this account, there has been an attempt to provide the temperature sensor at a portion that is not in contact with the belt on the surface of the heat-generating roller. In this case, however, the temperature sensor cannot accurately determine the amount of the heat removed from the belt and an appropriate amount of heating thus cannot be performed. On the other hand, when the temperature sensor is merely attached to the inner peripheral surface of the belt, accurate determination of the temperature is made difficult by variations in measured temperatures due to vibration or snaking of the belt.
If the heat-generating member is heated through electromagnetic induction while it is at rest, only a portion of the heat-generating member is extremely heated up, which may exceed the heat resistant temperature of the heat-generating member or any other members in contact with the heat-generating member. This may result in thermal alteration and thermal deformation of the member(s), which cause to degrade the quality of resultant images.
In the above-mentioned image heating device, only the operation signal to the drive motor is taken into consideration. Accordingly, the device is not capable of dealing with the trouble occurring in the path for transporting the driving force from the drive motor to the image heating device. Particularly, in the image heating device configured to be freely attachable/detachable to/from the image forming apparatus main body, insufficient installation, damage to the gear for transporting the driving force from the drive motor, and the like are liable to occur, which may lead to a problem that the heat-generating member does not rotate while the drive motor is rotating.